A captive panel fastener having considerable use in the aerospace industry, disclosed in U.S. Pat. No. 3,995,675, includes a screw with a head at one end and a shank projecting from the head. A thread is formed on the outer portion of the shank and three axial grooves extend most of the length of the shank from its outer end. A washer fits over the shank for retaining the fastener to a workpiece. Tabs on the inner edge of the washer extend into the grooves in the shank. At the outer end of the shank is a straight cylindrical bore into which is pressed the knurled shank of a plug. The plug has a head with a diameter such that it extends into the grooves in the shank, but the head is of lesser diameter than the root diameter of the thread. Therefore, the plug is held to the shank of the screw by the knurl and the head of the plug interferes with the tabs on the washer to prevent removal of the washer from the shank.
In use, the screw is extended through an opening in a panel and the washer and plug are applied afterward so that the plug holds the washer to the screw and the washer cooperates with the head of the screw to prevent removal of the screw from the panel.
Although it may be used effectively, this fastener has certain disadvantages. One is that it has been thought necessary to truncate the root of the thread so that the thread has a depth less than that of a standard thread. In theory this minimizes the depth necessary for the grooves so that the strength of the screw is not compromised. However, the truncated thread will not accept a conventional nut, requiring instead a special nut to mate with the screw. This adds to the expense of the fastener. Moreover, the truncated thread requires special dies, which are more expensive than and not as readily available as standard thread rolling dies. This increases the manufacturing cost of the fastener.
Another problem arises from the pressing of the plug into the bore formed in the end of the shank. Considerable force is necessary to accomplish this so that, in some instances, the head of the plug is not pressed fully into engagement with the outer end of the shank. Also, under certain tolerance conditions, the insertion of the knurled shank of the plug can cause expansion of the thread of the fastener at its outer end. This may make it impossible to mate the screw with a nut.
Once the plug is installed, it cannot be removed. In some instances, however, it is desirable to remove the plug so that the screw can be removed from a panel, reused or a different washer applied to it. With the prior art design, such steps are impossible and the entire fastener must be discarded if changes are to be made. Removal of the fastener from the panel becomes difficult because it can be accomplished only by breaking the retainer washer.